And finally, is fasting healthy? Although fasting has been practiced for thousands of years, the question is still a subject of intense medical debate. WebMD consulted experts on weight loss and fasting for some answers.

Fasting and Weight Loss

If you weed through all the controversy, you'll find that most medical experts agree on one thing: fasting is not a healthy weight loss tool.

"The appeal is that [fasting] is quick, but it is quick fluid loss, not substantial weight loss," says Madelyn Fernstrom, PhD, CNS, founder and director of the University of Pittsburgh Medical Center's Weight Loss Management Center.

"If it's easy off, it will come back quickly" -- as soon as you start eating normally again, she says.

Even some proponents of fasting for other medical purposes do not support fasting for weight loss. Some say it can actually make weight problems worse.

"Fasting is not a weight loss tool. Fasting slows your metabolic rate down so your diet from before the fast is even more fattening after you fast," says Joel Fuhrman MD, author of Eat to Live: The Revolutionary Plan for Fast and Sustained Weight Loss and Fasting and Eating for Health.

Fasting for weight loss carries other health risks as well.

While fasting for a day or two is rarely a problem if you are healthy, "it can be quite dangerous if you are not already eating a healthy diet, or if you've got liver or kidney problems, any kind of compromised immune system functioning, or are on medication -- even Tylenol," says Fuhrman, a family physician in Flemington, N.J..

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Even worse for dieters is that fasting for weight loss "distracts people from the real message of how to lose weight: lower fat intake, eat five fruits and vegetables a day, drink water and stop drinking other liquids, walk 30 minutes a day, and get more sleep," says Fernstrom, an associate professor of psychiatry, epidemiology, and surgery at the University of Pittsburgh School of Medicine.

In addition, other practices that are often combined with fasting for weight loss, such as colon cleansing, carry their own risks.

"Fasts are sometimes accompanied with enemas to cleanse your intestinal tract, and that can be very dangerous," says Fernstrom. "The intestinal tract has a lot of good bacteria. When you are changing that balance, the good bacteria are affected, too."

Does Fasting Detoxify the Body?

Here's where the debate gets intense.

"There is no scientific evidence it will detox the body. The issue of fasting to cleanse the body has no biological basis because the body is real good at that by itself," says Fernstrom. "The liver is a natural detox center; the lungs, the colon, the kidneys, [the lymph glands] and the skin get rid of toxins."

But Fuhrman, who has supervised hundreds of patients' fasts for medicinal purposes, disagrees.

"We know that the body is unable to rid itself of toxins when we eat a diet low in nutrients," and that applies to most Americans, even those who think they are healthy, he says.

"Americans eat 51% of their diet from processed foods and foods low in phytochemicals and antioxidants," he says. "So you see a buildup of waste products in the cells -- AGE, advanced glycation end products -- that build up in cellular tissues and lead to atherosclerosis, aging, diabetes, nerve damage, and the deterioration of organs. This is basic science and physiology every doctor learns in medical school."

Along with improving your overall diet, fasting is one solution to that buildup of AGE, according to advocates like Fuhrman.

"Fasting allows the body to most effectively remove these waste products," he says. "The body is designed to fast; we do it every night."

How does fasting remove toxins from the body? When you go without eating for more than a day or two, the body enters into ketosis. Ketosis occurs when the body runs out of carbohydrates to burn for energy, so it burns fat.

"And the fat is where the body stores many of the toxins it absorbs from the environment," Fuhrman says.

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Spiritual and Religious Fasting

Whether fasting can help rid the body of waste buildup is a matter of controversy. But fasting has been used for religious and spiritual purification for centuries.

Nearly every religious text you can name, from the Old and New Testaments of the Bible to the Quran and the Upanishads, calls upon followers to fast periodically as a rite of spiritual purification, penitence, or preparation for union with God.

Medical Reasons for Fasting

Another topic on which there is medical agreement is the benefit - actually, the necessity -- of fasting before surgery.

"You don't want the body to be digesting food as it manages the slower breathing [and other body changes] under anesthesia," says Fernstrom.

Fasting is also required to get accurate readings for certain medical tests. Short-term fasting before tests for cholesterol and blood sugar levels, for example, helps achieve a more accurate baseline count.

Fasting to Treat Disease

In his practice, Fuhrman tells WebMD, he has seen fasting -- combined with improving the diet before and afterward -- eliminate lupus, arthritis and chronic skin conditions like psoriasis and eczema. He says he has also seen fasting heal the digestive tracts of those with ulcerative colitis and Crohn's disease, and lower blood pressure.

"Fasting followed by a vegetarian diet interferes with the immune system's activities, especially if the immune system is overreacting, as it does with ," and other auto-immune diseases, he says. He cites half a dozen studies published in medical journals ranging from the American Journal of Physiology - Endocrinology and Metabolism to the Scandinavian Journal of Rheumatology.

Studies published in The Proceedings of the National Academy of Sciences and The Journal of Nutrition in 2003 showed that mice forced to fast every other day, while eating twice the normal amount of food on non-fasting days, had better insulin control, neuronal resistance to injury, and other health indicators than mice fed calorie-restricted diets.

Fasting may yield psychological benefits as well.

"I use very brief fasting with my patients to help them cope with stress and depression," says Agnese Barolo, a life coach in contemplative practices in New Rochelle, N.Y. "I start them with just a few hours â so they learn to say no to food. Itâs the first step in taking control of their lives."

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Many are so encouraged that they try longer fasts, she says.

"I know some doctors say there is no scientific evidence for the curative powers of fasting," Barolo says. "But there is a reason every culture in every country has practiced some form of fasting for thousands of years."Â

Fasting is not advisable for everyone. But for those whose medical conditions do not respond to other treatments, Fuhrman says, "sometimes fasting four to five days a month can help them break to a next level of immune competency."

He adds that "it will only work if you frame the fast with good nutrition before and after. For most medical conditions, if you stick to a strict diet, you donât even need fasting."